Chapter 28: MAGNETIC FIELDS 1. Units of a magnetic field might be: A. C·m/s B. C·s/m C. C/kg D. kg/C·s E. N/C·m ans: D 2. In the formula F = qv × B: A. F must be perpendicular to v but not necessarily to B B. F must be perpendicular to B but not necessarily to v C. v must be perpendicular to B but not necessarily to F D. all three vectors must be mutually perpendicular E. F must be perpendicular to both v and B ans: E 3. An electron moves in the negative x direction, through a uniform magnetic field in the negative y direction. The magnetic force on the electron is: y .. ... .. ... ... ... ... . ... ... .... ... .... ... .... ... ....... ... ..... .. ...................................................................................................................................... .. ... . . . .. ... . . . .. .... .... .... .... .... .. ... . . . ... .. . . .... . .. . . .. . .. . . . ... .... .... .. .. v ...........................•... ... . ........ . B x z A. B. C. D. E. 4. At A. B. C. D. E. in the negative x direction in the positive y direction in the negative y direction in the positive z direction in the negative z direction ans: E any point the magnetic field lines are in the direction of: the magnetic force on a moving positive charge the magnetic force on a moving negative charge the velocity of a moving positive charge the velocity of a moving negative charge none of the above ans: E Chapter 28: MAGNETIC FIELDS 405 5. The magnetic force on a charged particle is in the direction of its velocity if: A. it is moving in the direction of the field B. it is moving opposite to the direction of the field C. it is moving perpendicular to the field D. it is moving in some other direction E. never ans: E 6. A magnetic field exerts a force on a charged particle: A. always B. never C. if the particle is moving across the field lines D. if the particle is moving along the field lines E. if the particle is at rest ans: C 7. The direction of the magnetic field in a certain region of space is determined by firing a test charge into the region with its velocity in various directions in di erent trials. The field direction is: A. one of the directions of the velocity when the magnetic force is zero B. the direction of the velocity when the magnetic force is a maximum C. the direction of the magnetic force D. perpendicular to the velocity when the magnetic force is zero E. none of the above ans: A 8. An electron is moving north in a region where the magnetic field is south. The magnetic force exerted on the electron is: A. zero B. up C. down D. east E. west ans: A 9. A magnetic field CANNOT: A. exert a force on a charged particle B. change the velocity of a charged particle C. change the momentum of a charged particle D. change the kinetic energy of a charged particle E. change the trajectory of a charged particle ans: D 406 Chapter 28: MAGNETIC FIELDS 10. A proton (charge e), traveling perpendicular to a magnetic field, experiences the same force as an alpha particle (charge 2e) which is also traveling perpendicular to the same field. The ratio of their speeds, vproton /valpha , is: A. 0.5 B. 1 C. 2 D. 4 E. 8 ans: C 11. A hydrogen atom that has lost its electron is moving east in a region where the magnetic field is directed from south to north. It will be deflected: A. up B. down C. north D. south E. not at all ans: A 12. A beam of electrons is sent horizontally down the axis of a tube to strike a fluorescent screen at the end of the tube. On the way, the electrons encounter a magnetic field directed vertically downward. The spot on the screen will therefore be deflected: A. upward B. downward C. to the right as seen from the electron source D. to the left as seen from the electron source E. not at all ans: C 13. An electron (charge = 1.6 × 10 19 C) is moving at 3 × 105 m/s in the positive x direction. A magnetic field of 0.8 T is in the positive z direction. The magnetic force on the electron is: A. 0 B. 4 × 10 14 N, in the positive z direction C. 4 × 10 14 N, in the negative z direction D. 4 × 10 14 N, in the positive y direction E. 4 × 10 14 N, in the negative y direction ans: D 14. At one instant an electron (charge = 1.6×10 19 C) is moving in the xy plane, the components of its velocity being vx = 5 × 105 m/s and vy = 3 × 105 m/s. A magnetic field of 0.8 T is in the positive x direction. At that instant the magnitude of the magnetic force on the electron is: A. 0 B. 2.6 × 10 14 N C. 3.8 × 10 14 N D. 6.4 × 10 14 N E. 1.0 × 10 13 N ans: C Chapter 28: MAGNETIC FIELDS 407 15. At one instant an electron (charge = 1.6×10 19 C) is moving in the xy plane, the components of its velocity being vx = 5 × 105 m/s and vy = 3 × 105 m/s. A magnetic field of 0.8 T is in the positive x direction. At that instant the magnitude of the magnetic force on the electron is: A. 0 B. 3.8 × 10 14 N C. 5.1 × 10 14 N D. 6.4 × 10 14 N E. 7.5 × 10 14 N ans: B 16. An electron travels due north through a vacuum in a region of uniform magnetic field B that is also directed due north. It will: A. be una ected by the field B. speed up C. slow down D. follow a right-handed corkscrew path E. follow a left-handed corkscrew path ans: A 17. At one instant an electron is moving in the positive x direction along the x axis in a region where there is a uniform magnetic field in the positive z direction. When viewed from a point on the positive z axis, it subsequent motion is: A. straight ahead B. counterclockwise around a circle in the xy plane C. clockwise around a circle in the xy plane D. in the positive z direction E. in the negative z direction ans: B 18. A uniform magnetic field is directed into the page. A charged particle, moving in the plane of the page, follows a clockwise spiral of decreasing radius as shown. A reasonable explanation is: particle ................................. ......... ....... . ....... ...... .. ...... ............................... ................... .... . . . . . . . . ...... .. ... .......... .... ...... . . . . ... . . . . . . .. . . . . . .. .. . . . . . . . . . . ..... ... ... ..... .... ... ... ..... .... .... .... ................ .... .... .... ... ... ... ... ... ... ... ... . . . . ... .. .. .. .... .... .... .. .... .... . .... ... .... .. ... ... . . . . . .................... . ... . ... . . . . . . .. . ... ......... ..... .... ... ............................... ... ..... ..... ...... ...... . ....... . . . . . ............................. • B A. B. C. D. E. 408 the charge is positive and slowing down the charge is negative and slowing down the charge is positive and speeding up the charge is negative and speeding up none of the above ans: B Chapter 28: MAGNETIC FIELDS 19. An electron and a proton each travel with equal speeds around circular orbits in the same uniform magnetic field, as shown in the diagram (not to scale). The field is into the page on the diagram. Because the electron is less massive than the proton and because the electron is negatively charged and the proton is positively charged: • • ..................... ......... ...... ..... ... .... ... .. ... . ... ..... .. ... ... ... ... .. . . ... . . ... . . . . ..... ....... ..... ............................ B .............................. ............... ......... ........ ....... ....... ...... ...... ..... . . . . .... .... . .... . .. . ... . .. ... . . ... ... . ... .. ... . ... .... .. ... .. .. ... .. .. ... .. ... .. . ... .... ... ... ... . ... .. ... ... ... ... ... ... ... . .... .. .... ... ...... ..... ...... ...... ....... ...... . . . . . .......... . . .......................................... A. the electron travels clockwise around the smaller circle and the proton travels counterclockwise around the larger circle B. the electron travels counterclockwise around the smaller circle and the proton travels clockwise around the larger circle C. the electron travels clockwise around the larger circle and the proton travels counterclockwise around the smaller circle D. the electron travels counterclockwise around the larger circle and the proton travels clockwise around the smaller circle E. the electron travels counterclockwise around the smaller circle and the proton travels counterclockwise around the larger circle ans: A 20. An electron is launched with velocity v in a uniform magnetic field B. The angle between v and B is between 0 and 90 . As a result, the electron follows a helix, its velocity vector v returning to its initial value in a time interval of: A. 2 m/eB B. 2 mv/eB C. 2 mv sin /eB D. 2 mv cos /eB E. none of these ans: A 21. An electron and a proton are both initially moving with the same speed and in the same direction at 90 to the same uniform magnetic field. They experience magnetic forces, which are initially: A. identical B. equal in magnitude but opposite in direction C. in the same direction and di ering in magnitude by a factor of 1840 D. in opposite directions and di ering in magnitude by a factor of 1840 E. equal in magnitude but perpendicular to each other. ans: B Chapter 28: MAGNETIC FIELDS 409 22. An electron enters a region of uniform perpendicular E and B fields. It is observed that the velocity v of the electron is una ected. A possible explanation is: A. v is parallel to E and has magnitude E/B B. v is parallel to B C. v is perpendicular to both E and B and has magnitude B/E D. v is perpendicular to both E and B and has magnitude E/B E. the given situation is impossible ans: D 23. A charged particle is projected into a region of uniform, parallel, E and B fields. The force on the particle is: A. zero B. at some angle < 90 with the field lines C. along the field lines D. perpendicular to the field lines E. unknown (need to know the sign of the charge) ans: B 24. A uniform magnetic field is in the positive z direction. A positively charged particle is moving in the positive x direction through the field. The net force on the particle can be made zero by applying an electric field in what direction? A. Positive y B. Negative y C. Positive x D. Negative x E. Positive z ans: B 25. An electron is traveling in the positive x direction. A uniform electric field E is in the negative y direction. If a uniform magnetic field with the appropriate magnitude and direction also exists in the region, the total force on the electron will be zero. The appropriate direction for the magnetic field is: y v •.......................... . ........ .E A. B. C. D. E. 410 the positive y direction the negative y direction into the page out of the page the negative x direction ans: C Chapter 28: MAGNETIC FIELDS x 26. An ion with a charge of +3.2×10 19 C is in a region where a uniform electric field of 5×104 V/m is perpendicular to a uniform magnetic field of 0.8 T. If its acceleration is zero then its speed must be: A. 0 B. 1.6 × 104 m/s C. 4.0 × 104 m/s D. 6.3 × 104 m/s E. any value but 0 ans: D 27. The current is from left to right in the conductor shown. The magnetic field is into the page and point S is at a higher potential than point T. The charge carriers are: S • ..... ........................................................................... .... i • T A. B. C. D. E. positive negative neutral absent moving near the speed of light ans: A 28. Electrons (mass m, charge e) are accelerated from rest through a potential di erence V and are then deflected by a magnetic field B that is perpendicular to their velocity. The radius of the resulting electron trajectory is: A. ( 2eV /m)/B B. B 2eV /m C. ( 2mV /e)/B D. B 2mV /e E. none of these ans: C 29. In a certain mass spectrometer, an ion beam passes through a velocity filter consisting of mutually perpendicular fields E and B. The beam then enters a region of another magnetic field B perpendicular to the beam. The radius of curvature of the resulting ion beam is proportional to: A. EB /B B. EB/B C. BB /E D. B/EB E. E/BB ans: E Chapter 28: MAGNETIC FIELDS 411 30. A cyclotron operates with a given magnetic field and at a given frequency. If R denotes the radius of the final orbit, the final particle energy is proportional to: A. 1/R B. R C. R2 D. R3 E. R4 ans: C 31. J. J. Thomson’s experiment, involving the motion of an electron beam in mutually perpendicular E and B fields, gave the value of: A. mass of an electron B. charge of an electron C. Earth’s magnetic field D. charge/mass ratio for electrons E. Avogadro’s number ans: D 32. The diagram shows a straight wire carrying a flow of electrons into the page. The wire is between the poles of a permanent magnet. The direction of the magnetic force exerted on the wire is: N A. B. C. D. E. into the page ans: A 412 Chapter 28: MAGNETIC FIELDS .... ...... ....... .. ..... .... ..... ......... S 33. The figure shows the motion of electrons in a wire that is near the N pole of a magnet. The wire will be pushed: ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ••• ... ... ... • ... • ... • ... • • • • • ... • • • ... • • • • • • • ... • • • ... ••• ••••••••••• ... ... ••• ••••••• ..................... ... ... ... .. ... • . . . . • . .... ...... . . ... ... ••• .. ... ... ....... .............. ... ... . ... ....... . ... ....... ......... ....... ... . . . . . . . . . . . . ... ... ... .... ........... . . . . . . . . . . ... . . ... ...... .... ............. . . . ... . ........... . . ... ......... .. ... ... ....... .. . ... ...... . ...... .... ... ... ...................... ....... . ... ... ......................................... ... . . ... . . ...... .. ....... . . . . ... . ....... .... . . . . . . . . . . . . . . ... ........ .... .... . . . . . . . . . . ......... . . . .. ... ....... ....... ....... ................ ....... ........ ... ....... ..... .... ............ .... .... ....... ... .. ............ ......... ...... ........ .... A. B. C. D. E. electron flow toward the magnet away from the magnet downwards upwards along its length ans: D 34. The diagram shows a straight wire carrying current i in a uniform magnetic field. The magnetic force on the wire is indicated by an arrow but the magnetic field is not shown. Of the following possibilities, the direction of the magnetic field is: i .. ....................................................................... .. ... ... ... ... ... ... ... . .......... ....... ... A. B. C. D. E. F opposite the direction of the current opposite the direction of F in the direction of F into the page out of the page ans: E Chapter 28: MAGNETIC FIELDS 413 35. The figure shows a uniform magnetic field B directed to the left and a wire carrying a current into the page. The magnetic force acting on the wire is: .... ................................................................................................................................................................................................................... .... .... ................................................................................................................................................................................................................... .... B i .... ................................................................................................................................................................................................................... .... .... ................................................................................................................................................................................................................... .... A. B. C. D. E. toward toward toward toward zero ans: A the the the the top of the page bottom of the page left right 36. A loop of wire carrying a current of 2.0 A is in the shape of a right triangle with two equal sides, each 15 cm long. A 0.7 T uniform magnetic field is parallel to the hypotenuse. The resultant magnetic force on the two equal sides has a magnitude of: A. 0 B. 0.21 N C. 0.30 N D. 0.41 N E. 0.51 N ans: A 37. A loop of wire carrying a current of 2.0 A is in the shape of a right triangle with two equal sides, each 15 cm long. A 0.7 T uniform magnetic field is in the plane of the triangle and is perpendicular to the hypotenuse. The magnetic force on either of the two equal sides has a magnitude of: A. zero B. 0.105 N C. 0.15 N D. 0.21 N E. 0.25 N ans: C 38. A current is clockwise around the outside edge of this page and a uniform magnetic field is directed parallel to the page, from left to right. If the magnetic force is the only force acting on the page, the page will turn so the right edge: A. moves toward you B. moves away from you C. moves to your right D. moves to your left E. does not move ans: A 414 Chapter 28: MAGNETIC FIELDS 39. A square loop of wire lies in the plane of the page and carries a current I as shown. There is a uniform magnetic field B parallel to the side MK as indicated. The loop will tend to rotate: R . . K .......................................................................................... L ... ... ... ... .. .. .. .. . .. .. .. B .... ... .. .. .. ... .. .. .. ... Q .. .. P ... .. .. .. .. .. .. ......... .... .... I .. .. I ..... ... .. .. .. . .. .. .. ................................................................................ . . M N . . S A. B. C. D. E. about PQ with KL coming out of the page about PQ with KL going into the page about RS with MK coming out of the page about RS with MK going into the page about an axis perpendicular to the page. ans: A 40. The units of magnetic dipole moment are: A. ampere B. ampere·meter C. ampere·meter2 D. ampere/meter E. ampere/meter2 ans: C 41. You are facing a loop of wire which carries a clockwise current of 3.0 A and which surrounds an area of 5.8 × 10 2 m2 . The magnetic dipole moment of the loop is: A. 3.0 A · m2 , away from you B. 3.0 A · m2 , toward you C. 0.17 A · m2 , away from you D. 0.17 A · m2 , toward you E. 0.17 A · m2 , left to right ans: C 42. The magnetic torque exerted on a flat current-carrying loop of wire by a uniform magnetic field B is: A. maximum when the plane of the loop is perpendicular to B B. maximum when the plane of the loop is parallel to B C. dependent on the shape of the loop for a fixed loop area D. independent of the orientation of the loop E. such as to rotate the loop around the magnetic field lines ans: B Chapter 28: MAGNETIC FIELDS 415 43. A circular loop of wire with a radius of 20 cm lies in the xy plane and carries a current of 2 A, counterclockwise when viewed from a point on the positive z axis. Its magnetic dipole moment is: A. 0.25 A · m2 , in the positive z direction B. 0.25 A · m2 , in the negative z direction C. 2.5 A · m2 , in the positive z direction D. 2.5 A · m2 , in the negative z direction E. 0.25 A · m2 , in the xy plane ans: A 44. The diagrams show five possible orientations of a magnetic dipole µ in a uniform magnetic field B. For which of these does the magnetic torque on the dipole have the greatest magnitude? . µ ....... ....... ........................................ ................................................ B .. A µ ............................. .. ........................................................................................ .. B µ ......... .. ..... .... .. ........................................................................................ .. B B C ........ µ ...... ..... .. .. ........................................................................................ .. B D µ ............................ .. ........................................................................................ .. B E ans: A 45. The magnetic dipole moment of a current-carrying loop of wire is in the positive z direction. If a uniform magnetic field is in the positive x direction the magnetic torque on the loop is: A. 0 B. in the positive y direction C. in the negative y direction D. in the positive z direction E. in the negative z direction ans: B 46. For a loop of current-carrying wire in a uniform magnetic field the potential energy is a minimum if the magnetic dipole moment of the loop is: A. in the same direction as the field B. in the direction opposite to that of the field C. perpendicular to the field D. at an angle of 45 to the field E. none of the above ans: A 47. The diagrams show five possible orientations of a magnetic dipole µ in a uniform magnetic field B. For which of these is the potential energy the greatest? . µ ...... ........ ........................................ ............................................... B .. A µ ............................. .. ........................................................................................ .. B B ans: E 416 Chapter 28: µ ......... .. ..... .... .. ........................................................................................ .. MAGNETIC FIELDS C B ........ µ ...... ..... .. .. ........................................................................................ .. D B µ ............................ .. ........................................................................................ .. E B 48. A loop of current-carrying wire has a magnetic dipole moment of 5 × 10 4 A · m2 . The moment initially is aligned with a 0.5-T magnetic field. To rotate the loop so its dipole moment is perpendicular to the field and hold it in that orientation, you must do work of: A. 0 B. 2.5 × 10 4 J C. 2.5 × 10 4 J D. 1.0 × 10 3 J E. 1.0 × 10 3 J ans: B Chapter 28: MAGNETIC FIELDS 417 Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 1. Suitable units for µ0 are: A. tesla B. newton/ampere2 C. weber/meter D. kilogram·ampere/meter E. tesla·meter/ampere ans: E 2. A “coulomb” is: A. one ampere per second B. the quantity of charge that will exert a force of 1 N on a similar charge at a distance of 1 m C. the amount of current in each of two long parallel wires, separated by 1 m, that produces a force of 2 × 10 7 N/m D. the amount of charge that flows past a point in one second when the current is 1 A E. an abbreviation for a certain combination of kilogram, meter and second ans: D 3. Electrons are going around a circle in a counterclockwise direction as shown. At the center of the circle they produce a magnetic field that is: .......... ................ ........................... ....... ...... ...... .... . . . . . ... ... .... ... ... . ... .. . .. ... .... ... .............. ..... .... . ... .... ... ........... .. ... ... ... .. . . ... . ... ... .... ... ..... .... ...... ...... . . . ........ . . ........................................ A. B. C. D. E. 418 electron into the page out of the page to the left to the right zero ans: A Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 4. In the figure, the current element i d , the point P, and the three vectors (1, 2, 3) are all in the plane of the page. The direction of dB, due to this current element, at the point P is: 3... .. .... ....... .......... .. ... .. . ... ... .... . ... .................. ... ... .. . . ... . . ... .... .... ... ... ... .... ... . ... . . ... .... ... ... ... ... .... ... . ... . ... ....... .... ..... .. ................................................................................................................. .. i ................................................ ...... ... ... ... ... .... .... ....................................... . . . .. . ... ................................................... ..... d 2 • P A. B. C. D. E. 1 in the direction marked “1” in the direction marked “2” in the direction marked “3” out of the page into the page ans: E 5. The magnitude of the magnetic field at point P, at the center of the semicircle shown, is given by: .. i ................................... . . . . . . ..... ... ... .......... ... .. . .............. ... ... .... .. .... .. .. ........................................ R ....•... .................................... P A. B. C. D. E. 2µ0 i/R µ0 i/R µ0 i/4 R µ0 i/2R µ0 i/4R ans: E Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 419 6. The diagrams show three circuits consisting of concentric circular arcs (either half or quarter circles of radii r, 2r, and 3r) and radial lengths. The circuits carry the same current. Rank them according to the magnitudes of the magnetic fields they produce at C, least to greatest. .............................. ............ ....... ....... ...... ...... ..... ..... ..... . . . .... ... . . ... ... ... . . ... . . . ... .... ... ... ... ... .. .. ... ..................................... ... .................................................. .. . ... . . . ... . . . ..... ........................ 1 C • A. B. C. D. E. .............................. ............ ....... ....... ...... ...... ..... ..... ..... . . . .... ... . . ... ... ... . . ... . . . ... .... ... ........................ . . . ... . ... ... ... . . ... .. ... .. ... .... .. ..................................... . ................................................. .. 2 • C .............................. ............ ....... ....... ...... ...... ..... ..... ..... . . . .... ..... . . . . . ... . . . . . . . . . . ... .. ...... . ... . ... . . . . . ... .... ... . . . . . ... .. ... . . . .... ......... . ... ..... ... .... ... ... ... ... .. ... ... ... ....................................... .... .................... 3 • C 1, 2, 3 3, 2, 1 1, 3, 2 2, 3, 1 2, 1, 3 ans: B 7. Lines of the magnetic field produced by a long straight wire carrying a current are: A. in the direction of the current B. opposite to the direction of the current C. radially outward from the wire D. radially inward toward the wire E. circles that are concentric with the wire ans: E 8. In an overhead straight wire, the current is north. The magnetic field due to this current, at our point of observation, is: A. east B. up C. north D. down E. west ans: E 9. A wire carrying a large current i from east to west is placed over an ordinary magnetic compass. The end of the compass needle marked “N” will point: A. north B. south C. east D. west E. the compass will act as an electric motor, hence the needle will keep rotating ans: B 420 Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 10. The magnetic field outside a long straight current-carrying wire depends on the distance R from the wire axis according to: A. R B. 1/R C. 1/R2 D. 1/R3 E. 1/R3/2 ans: B 11. Which graph correctly gives the magnitude of the magnetic field outside an infinitely long straight current-carrying wire as a function of the distance r from the wire? B B . .. . .. ... . . .. .... . . . . . ........... r A B. ... ... ... .... .... ..... ...... .......... D B .. .... . . ... ..... . . . ... ..... . . . . ... r B r .......... ...... . . . ... ... . . ... .. . . r C B .. ..... ..... ..... ..... ..... ..... ..... .. r E ans: D 12. The magnetic field a distance 2 cm from a long straight current-carrying wire is 2.0 × 10 The current in the wire is: A. 0.16 A B. 1.0 A C. 2.0 A D. 4.0 A E. 25 A ans: C 5 T. 13. Two long parallel straight wires carry equal currents in opposite directions. At a point midway between the wires, the magnetic field they produce is: A. zero B. non-zero and along a line connecting the wires C. non-zero and parallel to the wires D. non-zero and perpendicular to the plane of the two wires E. none of the above ans: D Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 421 14. Two long straight wires are parallel and carry current in the same direction. The currents are 8.0 and 12 A and the wires are separated by 0.40 cm. The magnetic field in tesla at a point midway between the wires is: A. 0 B. 4.0 × 10 4 C. 8.0 × 10 4 D. 12 × 10 4 E. 20 × 10 4 ans: B 15. Two long straight wires are parallel and carry current in opposite directions. The currents are 8.0 and 12 A and the wires are separated by 0.40 cm. The magnetic field in tesla at a point midway between the wires is: A. 0 B. 4.0 × 10 4 C. 8.0 × 10 4 D. 12 × 10 4 E. 20 × 10 4 ans: E 16. Two long straight current-carrying parallel wires cross the x axis and carry currents I and 3I in the same direction, as shown. At what value of x is the net magnetic field zero? x 0 1 3 . ...... ........ .... A. B. C. D. E. 422 I 4 5 7 . ...... ........ .... 3I 0 1 3 5 7 ans: C Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 17. Two long straight wires pierce the plane of the paper at vertices of an equilateral triangle as shown below. They each carry 2 A, out of the paper. The magnetic field at the third vertex (P) has magnitude (in T): P .... ... ... ... .... ... ... . ... ... ... .. ... ... . ... . ... ... . ... .. . ... . . ... . .. ... . . ... ... . ... . . . ... ... ... . . ... . . .• .. .• . • • • • .• • • • • • • . • • • • .• • • • • • • • • • • • • • ..• .• • • • • • • • • • • .• .• ..• ......................................................................................• • • • • • • • • • • • • • 4 cm 2 A ••••••••••••••••••••••• A. B. C. D. E. 1.0 × 10 1.7 × 10 2.0 × 10 5.0 × 10 8.7 × 10 ans: B 4 cm 4 cm • 2A • • • • • • • • • • • • • • • • • • • • • • 5 5 5 6 6 18. The diagram shows three equally spaced wires that are perpendicular to the page. The currents are all equal, two being out of the page and one being into the page. Rank the wires according to the magnitudes of the magnetic forces on them, from least to greatest. ................. ... .. .... . .... ...... ......... ................. ... .. .... . .... ...... ......... ................. ... .. .... . .... ...... ......... 1 2 3 · A. B. C. D. E. × · 1, 2, 3 2, 1 and 3 tie 2 and 3 tie, then 1 1 and 3 tie, then 2 3, 2, 1 ans: B 19. Two parallel wires carrying equal currents of 10 A attract each other with a force of 1 mN. If both currents are doubled, the force of attraction will be: A. 1 mN B. 4 mN C. 0.5 mN D. 0.25 mN E. 2 mN ans: B Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 423 20. Two parallel long wires carry the same current and repel each other with a force F per unit length. If both these currents are doubled and the wire separation tripled, the force per unit length becomes: A. 2F/9 B. 4F/9 C. 2F/3 D. 4F/3 E. 6F ans: D 21. Two parallel wires, 4 cm apart, carry currents of 2 A and 4 A respectively, in the same direction. The force per unit length in N/m of one wire on the other is: A. 1 × 10 3 , repulsive B. 1 × 10 3 , attractive C. 4 × 10 5 , repulsive D. 4 × 10 5 , attractive E. none of these ans: D 22. Two parallel wires, 4 cm apart, carry currents of 2 A and 4 A respectively, in opposite directions. The force per unit length in N/m of one wire on the other is: A. 1 × 10 3 , repulsive B. 1 × 10 3 , attractive C. 4 × 10 5 , repulsive D. 4 × 10 5 , attractive E. none of these ans: C 23. Four long straight wires carry equal currents into the page as shown. The magnetic force exerted on wire F is: N W F A. B. C. D. E. 424 E S north east south west zero ans: B Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 24. A constant current is sent through a helical coil. The coil: A. tends to get shorter B. tends to get longer C. tends to rotate about its axis D. produces zero magnetic field at its center E. none of the above ans: A 25. The diagram shows three arrangements of circular loops, centered on vertical axes and carrying identical currents in the directions indicated. Rank the arrangements according to the magnitudes of the magnetic fields at the midpoints between the loops on the central axes. ............................................ ....... .... ... ... ..... .. .... ... . ....... . . . ............................................ ..... • A. B. C. D. E. ............................................ ....... .... ... ... ..... .. .... ... . ....... . . . ............................................ ..... • .......... ................... .......................... ........ ............................. ....... ...... ............. ...... .... .... .. .... ...... .. . ..... ..... ... .. ... .... .. .... . . .... ....... . . . ... . ... ...... . ......... ............................................. ............. ................................................. ...... • ........................................... ....... .... ... .... . ..... ... .... .... ....... . . . . . ......................................... ...... ........................................... ....... .... ... .... . ..... ... .... .... ....... . . . . . . ......................................... . . ... ........................................... ....... .... ... .... . ..... ... .... .... ....... . . . . . ......................................... ...... 1 2 3 1, 2, 3 2, 1, 3 2, 3, 1 3, 2, 1 3, 1, 2 ans: C 26. Helmholtz coils are commonly used in the laboratory because the magnetic field between them: A. can be varied more easily than the fields of other current arrangements B. is especially strong C. nearly cancels Earth’s magnetic field D. is parallel to the plane of the coils E. is nearly uniform ans: E 27. If the radius of a pair of Helmholtz coils is R then the distance between the coils is: A. R/4 B. R/2 C. R D. 2R E. 4R ans: C Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 425 28. If R is the distance from a magnetic dipole, then the magnetic field it produces is proportional to: A. R B. 1/R C. R2 D. 1/R2 E. 1/R3 ans: E 29. A square loop of current-carrying wire with edge length a is in the xy plane, the origin being at its center. Along which of the following lines can a charge move without experiencing a magnetic force? A. x = 0, y = a/2 B. x = a/2, y = a/2 C. x = a/2, y = 0 D. x = 0, y = 0 E. x = 0, z = 0 ans: D 30. In Ampere’s law, B · ds = µ0 i, the integration must be over any: A. surface B. closed surface C. path D. closed path E. closed path that surrounds all the current producing B ans: D 31. In Ampere’s law, B · ds = µ0 i, the symbol ds is: A. an infinitesimal piece of the wire that carries current i B. in the direction of B C. perpendicular to B D. a vector whose magnitude is the length of the wire that carries current i E. none of the above ans: E 32. In Ampere’s law, B · ds = µ0 i, the direction of the integration around the path: A. must be clockwise B. must be counterclockwise C. must be such as to follow the magnetic field lines D. must be along the wire in the direction of the current E. none of the above ans: E 426 Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 33. A long straight wire carrying a 3.0 A current enters a room through a window 1.5 m high and 1.0 m wide. The path integral B · ds around the window frame has the value (in T·m): A. 0.20 B. 2.5 × 10 7 C. 3.0 × 10 7 D. 3.8 × 10 6 E. none of these ans: D 34. Two long straight wires enter a room through a door. One carries a current of 3.0 A into the room while the other carries a current of 5.0 A out. The magnitude of the path integral B · ds around the door frame is: A. 2.5 × 10 6 T · m B. 3.8 × 10 6 T · m C. 6.3 × 10 6 T · m D. 1.0 × 10 5 T · m E. none of these ans: A 35. If the magnetic field B is uniform over the area bounded by a circle with radius R, the net current through the circle is: A. 0 B. 2 RB/µ0 C. R2 B/µ0 D. RB/2µ0 E. 2RB/µ0 ans: A ˆ where r is the position vector of the 36. The magnetic field at any point is given by B = Ar × k, point and A is a constant. The net current through a circle of radius R, in the xy plane and centered at the origin is given by: A. AR2 /µ0 B. 2 AR/µ0 C. 4 AR3 /3µ0 D. 2 AR2 /µ0 E. AR2 /2µ0 ans: D Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 427 37. A hollow cylindrical conductor (inner radius = a, outer radius = b) carries a current i uniformly spread over its cross section. Which graph below correctly gives B as a function of the distance r from the center of the cylinder? B B ............................... ... ... ... ... ... ... ... .................................. .. ... .. ... ... .. ... .. ... .. .... .. ........ r a b B r a A b ......... .... ..... . . . ... .. ... ... ... .... .. ...... .. ..... r a b B B ..... ... .... . . ... ..... ... ... . . . ... ... .... . . . ........ ... r a b D C B ..................................... ... ... . . . ... ... . ... . . . ... .. . .... . .. ....... . . ... r .. a b E ans: C 38. A long straight cylindrical shell carries current i parallel to its axis and uniformly distributed over its cross section. The magnitude of the magnetic field is greatest: A. at the inner surface of the shell B. at the outer surface of the shell C. inside the shell near the middle D. in hollow region near the inner surface of the shell E. near the center of the hollow region ans: B 39. A long straight cylindrical shell has inner radius Ri and outer radius Ro . It carries current i, uniformly distributed over its cross section. A wire is parallel to the cylinder axis, in the hollow region (r < Ri ). The magnetic field is zero everywhere outside the shell (r > Ro ). We conclude that the wire: A. is on the cylinder axis and carries current i in the same direction as the current in the shell B. may be anywhere in the hollow region but must be carrying current i in the direction opposite to that of the current in the shell C. may be anywhere in the hollow region but must be carrying current i in the same direction as the current in the shell D. is on the cylinder axis and carries current i in the direction opposite to that of the current in the shell E. does not carry any current ans: D 428 Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 40. A long straight cylindrical shell has inner radius Ri and outer radius Ro . It carries a current i, uniformly distributed over its cross section. A wire is parallel to the cylinder axis, in the hollow region (r < Ri ). The magnetic field is zero everywhere in the hollow region. We conclude that the wire: A. is on the cylinder axis and carries current i in the same direction as the current in the shell B. may be anywhere in the hollow region but must be carrying current i in the direction opposite to that of the current in the shell C. may be anywhere in the hollow region but must be carrying current i in the same direction as the current in the shell D. is on the cylinder axis and carries current i in the direction opposite to that of the current in the shell E. does not carry any current ans: E 41. The magnetic field B inside a long ideal solenoid is independent of: A. the current B. the core material C. the spacing of the windings D. the cross-sectional area of the solenoid E. the direction of the current ans: D 42. Two long ideal solenoids (with radii 20 mm and 30 mm, respectively) have the same number of turns of wire per unit length. The smaller solenoid is mounted inside the larger, along a common axis. The magnetic field within the inner solenoid is zero. The current in the inner solenoid must be: A. two-thirds the current in the outer solenoid B. one-third the current in the outer solenoid C. twice the current in the outer solenoid D. half the current in the outer solenoid E. the same as the current in the outer solenoid ans: E 43. Magnetic field lines inside the solenoid shown are: ......................................... ............. ....... ....... .... .... ... ..... . .... ........... ..... .... . . . . . . . . . . . . . . . . . . . . ............................................... ............................................................................................................................ ....... .. . ...... ....... ..... ....... ...... ... ..... ... ... ....... .................... . ....... . ....... ..................... .................................. ............... ...... ......................... .. . . . ... .. ... .. . .... ......................................................................... .......... .. ... ... ............ ........ ........ .... . . . . . . . . . . . . . . . . . . . . .................................................................................... ................................... ................................. .. .... ... .. .. ..... .. ..... ... . . ........ . . . . ..................... ....................... .......... I I A. B. C. D. E. clockwise circles as one looks down the axis from the top of the page counterclockwise circles as one looks down the axis from the top of the page toward the top of the page toward the bottom of the page in no direction since B = 0 ans: C Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS 429 44. Solenoid 2 has twice the radius and six times the number of turns per unit length as solenoid 1. The ratio of the magnetic field in the interior of 2 to that in the interior of 1 is: A. 2 B. 4 C. 6 D. 1 E. 1/3 ans: C 45. A solenoid is 3.0 cm long and has a radius of 0.50 cm. It is wrapped with 500 turns of wire carrying a current of 2.0 A. The magnetic field at the center of the solenoid is: A. 9.9 × 10 8 T B. 1.3 × 10 3 T C. 4.2 × 10 2 T D. 16 T E. 20 T ans: C 46. A toroid with a square cross section carries current i. The magnetic field has its largest magnitude: A. at the center of the hole B. just inside the toroid at its inner surface C. just inside the toroid at its outer surface D. at any point inside (the field is uniform) E. none of the above ans: B 47. A toroid has a square cross section with the length of an edge equal to the radius of the inner surface. The ratio of the magnitude of the magnetic field at the inner surface to the magnitude of the field at the outer surface is: A. 1/4 B. 1/2 C. 1 D. 2 E. 4 ans: D 430 Chapter 29: MAGNETIC FIELDS DUE TO CURRENTS Chapter 30: INDUCTION AND INDUCTANCE 1. The normal to a certain 1-m2 area makes an angle of 60 with a uniform magnetic field. The magnetic flux through this area is the same as the flux through a second area that is perpendicular to the field if the second area is: A. 0.866 m2 B. 1.15 m2 C. 0.5 m2 D. 2 m2 E. 1 m2 ans: C 2. Suppose this page is perpendicular to a uniform magnetic field and the magnetic flux through it is 5 Wb. If the page is turned by 30 around an edge the flux through it will be: A. 2.5 Wb B. 4.3 Wb C. 5 Wb D. 5.8 Wb E. 10 Wb ans: B 3. A 2-T uniform magnetic field makes an angle of 30 with the z axis. The magnetic flux through a 3-m2 portion of the xy plane is: A. 2.0 Wb B. 3.0 Wb C. 5.2 Wb D. 6.0 Wb E. 12 Wb ans: C 4. A uniform magnetic field makes an angle of 30 with the z axis. If the magnetic flux through a 1-m2 portion of the xy plane is 5 Wb then the magnetic flux through a 2-m2 portion of the same plane is: A. 2.5 Wb B. 4.3 Wb C. 5 Wb D. 5.8 Wb E. 10 Wb ans: E 5. 1 weber is the same as: A. 1 V/s B. 1 T/s C. 1 T/m D. 1 T · m2 2 E. 1 T/m ans: D Chapter 30: INDUCTION AND INDUCTANCE 431 6. 1 weber is the same as: A. 1 V · s B. 1 T · s C. 1 T/m D. 1 V/s E. 1 T/m2 ans: A 7. The units of motional emf are: A. volt/second B. volt·meter/second C. volt/tesla D. tesla/second E. tesla·meter2 /second ans: E 8. Faraday’s law states that an induced emf is proportional to: A. the rate of change of the magnetic field B. the rate of change of the electric field C. the rate of change of the magnetic flux D. the rate of change of the electric flux E. zero ans: C 9. The emf that appears in Faraday’s law is: A. around a conducting circuit B. around the boundary of the surface used to compute the magnetic flux C. throughout the surface used to compute the magnetic flux D. perpendicular to the surface used to compute the magnetic flux E. none of the above ans: B 10. If the magnetic flux through a certain region is changing with time: A. energy must be dissipated as heat B. an electric field must exist at the boundary C. a current must flow around the boundary D. an emf must exist around the boundary E. a magnetic field must exist at the boundary ans: D 432 Chapter 30: INDUCTION AND INDUCTANCE 11. A square loop of wire lies in the plane of the page. A decreasing magnetic field is directed into the page. The induced current in the loop is: A. counterclockwise B. clockwise C. zero D. up the left edge and from right to left along the top edge E. through the middle of the page ans: B 12. As an externally generated magnetic field through a certain conducting loop increases in magnitude, the field produced at points inside the loop by the current induced in the loop must be: A. increasing in magnitude B. decreasing in magnitude C. in the same direction as the applied field D. directed opposite to the applied field E. perpendicular to the applied field ans: D 13. At any instant of time the total magnetic flux through a stationary conducting loop is less in magnitude than the flux associated with an externally applied field. This might occur because: A. the applied field is normal to the loop and increasing in magnitude B. the applied field is normal to the loop and decreasing in magnitude C. the applied field is parallel to the plane of the loop and increasing in magnitude D. the applied field is parallel to the plane of the loop and decreasing in magnitude E. the applied field is tangent to the loop ans: A 14. A long straight wire is in the plane of a rectangular conducting loop. The straight wire carries a constant current i, as shown. While the wire is being moved toward the rectangle the current in the rectangle is: i A. B. C. D. E. .. ...... ... ... zero clockwise counterclockwise clockwise in the left side and counterclockwise in the right side counterclockwise in the left side and clockwise in the right side ans: C Chapter 30: INDUCTION AND INDUCTANCE 433 15. A long straight wire is in the plane of a rectangular conducting loop. The straight wire carries an increasing current in the direction shown. The current in the rectangle is: i A. B. C. D. E. ..... .. .. .. .. zero clockwise counterclockwise clockwise in the left side and counterclockwise in the right side counterclockwise in the left side and clockwise in the right side ans: C 16. A long straight wire is in the plane of a rectangular conducting loop. The straight wire initially carries a constant current i in the direction shown. While the current i is being shut o , the current in the rectangle is: i A. B. C. D. E. 434 .... ..... .. .. zero clockwise counterclockwise clockwise in the left side and counterclockwise in the right side counterclockwise in the left side and clockwise in the right side ans: B Chapter 30: INDUCTION AND INDUCTANCE 17. A rectangular loop of wire is placed midway between two long straight parallel conductors as shown. The conductors carry currents i 1 and i2 , as indicated. If i1 is increasing and i2 is constant, then the induced current in the loop is: ..... i1 ........ A. B. C. D. E. ... ..... ... .. i2 zero clockwise counterclockwise depends on i1 i2 depends on i1 + i2 ans: C 18. You push a permanent magnet with its north pole away from you toward a loop of conducting wire in front of you. Before the north pole enters the loop the current in the loop is: A. zero B. clockwise C. counterclockwise D. to your left E. to your right ans: C 19. A vertical bar magnet is dropped through the center of a horizontal loop of wire, with its north pole leading. At the instant when the midpoint of the magnet is in the plane of the loop, the induced current at point P, viewed from above, is: A. maximum and clockwise B. maximum and counterclockwise C. not maximum but clockwise D. not maximum but counterclockwise E. essentially zero ans: E 20. A circular loop of wire rotates about a diameter in a magnetic field that is perpendicular to the axis of rotation. Looking in the direction of the field at the loop the induced current is: A. always clockwise B. always counterclockwise C. clockwise in the lower half of the loop and counterclockwise in the upper half D. clockwise in the upper half of the loop and counterclockwise in the lower half E. sometimes clockwise and sometimes counterclockwise ans: E Chapter 30: INDUCTION AND INDUCTANCE 435 21. In the experiment shown: .... S.............. ....................................... .. ........ ........ ........ ..... ... .............................• .• ............................................. • ... .. ... .. ... .. • . .... ...... ...... ...... ... . ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... .... . . ............................................................................................................................................................................................ .. ... .. .. A. B. C. D. E. ......... ......... ......... ......... •......... ...................... ...................... ...................... •.................................................. ...... .. ... ... ... .. ... ......... . . . . . . ... . . ... ... . ... ... . .. .. ... . ... .. ... ... .. . ... . . ...... ...... .......... ... ... ... ... ... ... ... ... .. ......................................................................................................................... G there is a steady reading in G as long as S is closed a motional emf is generated when S is closed the current in the battery goes through G there is a current in G just after S is opened or closed since the two loops are not connected, the current in G is always zero ans: D 22. The emf developed in a coil X due to the current in a neighboring coil Y is proportional to the: A. magnetic field in X B. rate of change of magnetic field in X C. resistance of X D. thickness of the wire in X E. current in Y ans: B 23. One hundred turns of insulated copper wire are wrapped around an iron core of cross-sectional area 0.100 m2 . The circuit is completed by connecting the coil to a 10- resistor. As the magnetic field along the coil axis changes from 1.00 T in one direction to 1.00 T in the other direction, the total charge that flows through the resistor is: A. 10 2 C B. 2 × 10 2 C C. 1 C D. 2 C E. 0.20 C ans: D 436 Chapter 30: INDUCTION AND INDUCTANCE 24. In the circuit shown, there will be a non-zero reading in galvanometer G: ............................................................................................................................. .. .. .... ... ... . .... ......... ... .. ... . ... ......................... ........ .. .... . . . . . . . . . . . . . ..................... ... .. .. ................................ .... .. .. ... ... ........ . .... .. .... . . . . . .... .. ... .... .... . ... ...................................... ... ............................ .. • • A. B. C. D. E. S • • ..................................................................................................... .. .. .... ... ... . ... .... . . . . ... . . ... .... . .......... ................. . . . . . . .... .... . ................... . ... .. ................. . .. ... .................. ... ... ... ............. ... ..... . .................... .................. ... ... ..... ... ..... ... .... ... .. ... .. .......................................................................... .... • G • only just after S is closed only just after S is opened only while S is kept closed never only just after S is opened or closed ans: E 25. A magnet moves inside a coil. Consider the following factors: I. strength of the magnet II. number of turns in the coil III. speed at which the magnet moves Which can a ect the emf induced in the coil? A. I only B. II only C. III only D. I and II only E. I, II, III ans: E 26. The graph shows the magnitude B of a uniform magnetic field that is perpendicular to the plane of a conducting loop. Rank the five regions indicated on the graph according to the magnitude of the emf induced in the loop, from least to greatest. B 2 ......................................................... . . ..... ... ..... 3 ... . ..... . .. ..... . . 1.... ..... . ..... . .. ..... . . . . ....... .. ........ 4 . . . . ........ .. ........ . . ........ . .... ... t A. B. C. D. E. 1, 2, 3, 2, 4, 3, 4, 3, 1, 1, 3, 4, 4, 3, 2, ans: B 4 1 2 2 1 Chapter 30: INDUCTION AND INDUCTANCE 437 27. The circuit shown is in a uniform magnetic field that is into the page. The current in the circuit is 0.20 A. At what rate is the magnitude of the magnetic field changing? Is it increasing or decreasing?: 12 cm | | 12 cm | | .. .. .. ...................................................................................................... .. ... ... .. .. .. ... ... ... ... .... .... ... ... ... ... ... ... ... ... ... ... ..... ..... ... ... ... ... .... .... ... ... ... ... ... ... ... ... . ... .......................................... ........................................................ 10 4V A. B. C. D. E. zero 140 T/s, 140 T/s, 420 T/s, 420 T/s, ans: B decreasing increasing decreasing increasing 28. A changing magnetic field pierces the interior of a circuit containing three identical resistors. Two voltmeters are connected to the same points, as shown. V1 reads 1 mV. V2 reads: ...................... .... ... .. .......................................... .... .......................................................... ... .... .... .... 2 ..... .................. ... ... .... ... .. .. ... ... .... .... .. .. . . . . . . . . . . . . . . . . . . . . . ....................................................................... ....... ....... ................................. ...... ....... .............................................................. .. .. .. .. ... .. .. .. ... .. . .... ... ... . . ... ... . ... ... ..... .... ... ... . . ... ... .... ... .... ... ... .... .... ... ... .. .. ... .. ... . . . . . . . . . . . . ... ........................................................ ....... ....... ............................................... . .... .. .. .. ... ... ... ... ... ... ... ... ... ... ... ... ... . . . . . . . . . . . . ..... ... ... ..... . . . . ... .. ................................................................................. .... . .......................................................................................................... ... . ... 1 . . ..... .................. V R R • • R V A. B. C. D. E. 438 0 1/3 mV 1/2 mV 1 mV 2 mV ans: E Chapter 30: INDUCTION AND INDUCTANCE 29. A circular loop of wire is positioned half in and half out of a square region of constant uniform magnetic field directed into the page, as shown. To induce a clockwise current in this loop: y • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • B A. B. C. D. E. x loop move it in +x direction move it in +y direction move it in y direction move it in x direction increase the strength of the magnetic field ans: A 30. The four wire loops shown have edge lengths of either L, 2L, or 3L. They will move with the same speed into a region of uniform magnetic field B, directed out of the page. Rank them according to the maximum magnitude of the induced emf, least to greatest. ..... ....................................................................... .... 1 A. B. C. D. E. 2 3 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 and 2 tie, then 3 and 4 tie 3 and 4 tie, then 1 and 2 tie 4, 2, 3, 1 1, then 2 and 3 tie, then 4 1, 2, 3, 4 ans: D Chapter 30: INDUCTION AND INDUCTANCE 439 31. A square loop of wire moves with a constant speed v from a field-free region into a region of constant uniform magnetic field, as shown. Which of the five graphs correctly shows the induced current i in the loop as a function of time t? v • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • . • • • • ................................................ .... • • • • • .. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • i . ......... . . . ... ... ... .. A ..... .. ..... .. ..... .. .... i i t ... ... .... . . . . ... .. ... .. B ...................... ... .. .. .. ... .. B D i ... .... ... ... ... .... ...... i ...................... ... .. .. .. ... .. t t ...................... ... .. .. .. ... .. E .............................. . . .... ... .... ... . .... . . ... C ... ... ... .. ... . .. .................... t t ans: C 32. The figure shows a bar moving to the right on two conducting rails. To make an induced current i in the direction indicated, a constant magnetic field in region A should be in what direction? i . .......................................................... .. i ... ... ... ... ... . .......... ........ .. A i ....... .......... ... ... ... .. ... v .. .................................................. .. .. .................................................. .. i A. B. C. D. E. 440 Right Left Into the page Out of the page Impossible; this cannot be done with a constant magnetic field ans: C Chapter 30: INDUCTION AND INDUCTANCE 33. A car travels northward at 75 km/h along a straight road in a region where Earth’s magnetic field has a vertical component of 0.50 × 10 4 T. The emf induced between the left and right side, separated by 1.7 m, is: A. 0 B. 1.8 mV C. 3.6 mV D. 6.4 mV E. 13 mV ans: B 34. Coils P and Q each have a large number of turns of insulated wire. When switch S is closed, the pointer of galvanometer G is deflected toward the left. With S now closed, to make the pointer of G deflect toward the right one could: .......................... ... ... .... .... ... ... .... .... .... .... .. .. .. .. . . ... ... .. .. .. .. ... ... .... .... .. .. ... ... ... ... ... ... ... ... .. ... . ... ... .. ... . ... ... .. .. ... ... .... .... ... ... ... ... .... ...... ... ................. .... .... ... ... .. ... ......................................................................................................................................... .... ... ... ... .. ... ... ... ......... ... ......... .. . . . . . . . . ..... . ... ...... ....... ....... . . . . ... ... ... ... ... ... ............................................... .. ...................................... ..... ..... ..... P • • S R A. B. C. D. E. .......................... ... ... .... .... ... ... .... .... .... .... .. .. .. .. . . ... ... .. .. .. .. ... ... .... .... .. .. ... ... ... ... ... ... ... ... .. ... . ... ... .. ... . ... ... .. .. ... ... .... .... ... ... ... ... .... ...... ... ................ .... .... ... ... ... ... ...................................................................................................................................................................... ... .. ... ... ... ... ... ... ... ... .................... . ... ... ... . ........................................................ .... ........................................................ .. ... . .... . . . .............. Q G move the slide of the rheostat R quickly to the right move coil P toward coil Q move coil Q toward coil P open S do none of the above ans: D 35. A rod lies across frictionless rails in a constant uniform magnetic field B, as shown. The rod moves to the right with speed v. In order for the emf around the circuit to be zero, the magnitude of the magnetic field should: ...... ........................... v .......................................................... ........................ .. ................................................. ..................... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. .... .. .. . .. .. ................................................ ....................... ........................................................ ..................... ...... A. B. C. D. E. not change increase linearly with time decrease linearly with time increase quadratically with time decrease quadratically with time ans: C Chapter 30: INDUCTION AND INDUCTANCE 441 36. A rectangular loop of wire has area A. It is placed perpendicular to a uniform magnetic field B and then spun around one of its sides at frequency f . The maximum induced emf is: A. BAf B. BAf C. 2BAf D. 2 BAf E. 4 BAf ans: D 37. A rectangular loop of wire is placed perpendicular to a uniform magnetic field and then spun around one of its sides at frequency f . The induced emf is a maximum when: A. the flux is zero B. the flux is a maximum C. the flux is half its maximum value D. the derivative of the flux with respect to time is zero E. none of the above ans: A 38. The diagram shows a circular loop of wire that rotates at a steady rate about a diameter O that is perpendicular to a uniform magnetic field. The maximum induced emf occurs when the point X on the loop passes: e • ..................................................................................................................................................................................... • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •• • • • • • • ............................................................................................• ..............• ........................................................................... • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • ............................................................................................• ..............• ........................................................................... • • • • • • • • • • • • • • • • • • • • • • • • • • • • • ............................................................................................• ..............• ........................................................................... • • • • • •• • • • • • • • • • •• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • d• c• • b •O • a X ..................................................................................................................................................................................... A. B. C. D. E. ........ ... ........ ........ ... ........ ........ ... ........ B ........ ... ........ ........ ... ........ a b c d e ans: C 39. A copper hoop is held in a vertical east-west plane in a uniform magnetic field whose field lines run along the north-south direction. The largest induced emf is produced when the hoop is: A. rotated about a north-south axis B. rotated about an east-west axis C. moved rapidly, without rotation, toward the east D. moved rapidly, without rotation, toward the south E. moved rapidly, without rotation, toward the northwest ans: B 442 Chapter 30: INDUCTION AND INDUCTANCE 40. A 10-turn conducting loop with a radius of 3.0 cm spins at 60 revolutions per second in a magnetic field of 0.50 T. The maximum emf generated is: A. 0.014 V B. 0.53 V C. 5.3 V D. 18 V E. 180 V ans: C 41. A single loop of wire with a radius of 7.5 cm rotates about a diameter in a uniform magnetic field of 1.6 T. To produce a maximum emf of 1.0 V, it should rotate at: A. 0 B. 2.7 rad/s C. 5.6 rad/s D. 35 rad/s E. 71 rad/s ans: D 42. A merry-go-round has an area of 300 m2 and spins at 2 rpm about a vertical axis at a place where Earth’s magnetic field is vertical and has a magnitude of 5 × 10 5 T. The emf around the rim is: A. 0 B. 0.5 mV C. 3.1 mV D. 15 mV E. 30 mV ans: A Chapter 30: INDUCTION AND INDUCTANCE 443 43. A copper penny slides on a horizontal frictionless table. There is a square region of constant uniform magnetic field perpendicular to the table, as shown. Which graph correctly shows the speed v of the penny as a function of time t? top view • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • .. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • .... • • • • • • • • • • • • ......................................... • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • . . • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • v • v B v ......................................................................... v ................ ............. ... .... .................................. t ............... ............... ............. ............. . .............. t A t B v C v ...................... ..................... ... .................... .................. ...... ....... ....... ............................ t t D E ans: D 44. A rod with resistance R lies across frictionless conducting rails in a constant uniform magnetic field B, as shown. Assume the rails have negligible resistance. The magnitude of the force that must be applied by a person to pull the rod to the right at constant speed v is: x ........ .................................. . ............................................................................ ............................v... .. .. . .. ................................................................... ............................ .. .. .. ... | . . .. ... .. .. .. ... . . L . . .. ... .. .. .. ... | . . . .. .. .. .. .................................................................. .............................. ............................................................................. ............................. ....... A. B. C. D. E. 444 0 BLv BLv/R B 2 L2 v/R B 2 Lxv/R ans: D Chapter 30: INDUCTION AND INDUCTANCE 45. A rod of length L and electrical resistance R moves through a constant uniform magnetic field B, perpendicular to the rod. The force that must be applied by a person to keep the rod moving with constant velocity v is: A. 0 B. BLv C. BLv/R D. B 2 L2 v/R E. B 2 L2 v 2 /R ans: A 46. As a loop of wire with a resistance of 10 moves in a constant non-uniform magnetic field, it loses kinetic energy at a uniform rate of 4.0 mJ/s. The induced current in the loop: A. is 0 B. is 2 mA C. is 2.8 mA D. is 20 mA E. cannot be calculated from the given data ans: D 47. As a loop of wire with a resistance of 10 moves in a non-uniform magnetic field, it loses kinetic energy at a uniform rate of 5 mJ/s. The induced emf in the loop: A. is 0 B. is 0.2 V C. is 0.28 V D. is 2 V E. cannot be calculated from the given data ans: B 48. An A. B. C. D. E. electric field is associated with every: magnetic field time-dependent magnetic field time-dependent magnetic flux object moving in a magnetic field conductor moving in a magnetic field ans: B 49. A cylindrical region of radius R = 3.0 cm contains a uniform magnetic field parallel to its axis. If the electric field induced at a point R/2 from the cylinder axis is 4.5 × 10 3 V/m the magnitude of the magnetic field must be changing at the rate: A. 0 B. 0.30 T/s C. 0.60 T/s D. 1.2 T/s E. 2.4 T/s ans: C Chapter 30: INDUCTION AND INDUCTANCE 445 50. A cylindrical region of radius R contains a uniform magnetic field parallel to its axis. The field is zero outside the cylinder. If the magnitude of the field is changing at the rate dB/dt, the electric field induced at a point 2R from the cylinder axis is: A. zero B. 2R dB/dt C. R dB/dt D. (R/2) dB/dt E. (R/4) dB/dt ans: E 51. A cylindrical region of radius R contains a uniform magnetic field, parallel to its axis, with magnitude that is changing linearly with time. If r is the radial distance from the cylinder axis, the magnitude of the induced electric field inside the cylinder is proportional to: A. R B. r C. r 2 D. 1/r E. 1/r2 ans: B 52. A cylindrical region of radius R contains a uniform magnetic field, parallel to its axis, with magnitude that is changing linearly with time. If r is the radial distance from the cylinder axis, the magnitude of the induced electric field outside the cylinder is proportional to: A. R B. r C. r 2 D. 1/r E. 1/r2 ans: D 53. The unit “henry” is equivalent to: A. volt·second/ampere B. volt/second C. ohm D. ampere·volt/second E. ampere·second/volt ans: A 446 Chapter 30: INDUCTION AND INDUCTANCE 54. The diagram shows an inductor that is part of a circuit. The direction of the emf induced in the inductor is indicated. Which of the following is possible? E ....... ....... ....... ....... .. ... .. ... .. ... .. ... .. ... .. ... .. ... .. ... ....................................................... .. .... . ... .... ... .... ... .... ... ........................................................ ... ... .. ... .. ... .. ... ... ... ... ... ... ... ... ... ... ... ... ... . . . ...... ..................................................................................................................... A. B. C. D. E. The current is constant and rightward The current is constant and leftward The current is increasing and rightward The current is increasing and leftward None of the above ans: D 55. A 10-turn ideal solenoid has an inductance of 3.5 mH. When the solenoid carries a current of 2.0 A the magnetic flux through each turn is: A. 0 B. 3.5 × 10 4 wb C. 7.0 × 10 4 wb D. 7.0 × 10 3 wb E. 7.0 × 10 2 wb ans : C 56. A 10-turn ideal solenoid has an inductance of 4.0 mH. To generate an emf of 2.0 V the current should change at a rate of: A. zero B. 5.0 A/s C. 50 A/s D. 250 A/s E. 500 A/s ans: E 57. A long narrow solenoid has length area A. Its inductance is: A. µ0 N 2 A B. µ0 N 2 A/ C. µ0 N A/ D. µ0 N 2 /A E. none of these ans: B and a total of N turns, each of which has cross-sectional Chapter 30: INDUCTION AND INDUCTANCE 447 58. A flat coil of wire, having 5 turns, has an inductance L. The inductance of a similar coil having 20 turns is: A. 4L B. L/4 C. 16L D. L/16 E. L ans: C 59. An inductance L, resistance R, and ideal battery of emf E are wired in series. A switch in the circuit is closed at time 0, at which time the current is zero. At any later time t the current i is given by: A. (E/R)(1 e Lt/R ) B. (E/R)e Lt/R C. (E/R)(1 + e Rt/L ) D. (E/R)e Rt/L E. (E/R)(1 e Rt/L ) ans: E 60. An inductance L, resistance R, and ideal battery of emf E are wired in series. A switch in the circuit is closed at time 0, at which time the current is zero. At any later time t the emf of the inductor is given by: A. E(1 e Lt/R ) B. Ee Lt/R C. E(1 + e Rt/L ) D. Ee Rt/L E. E(1 e Rt/L ) ans: D 61. An inductance L, resistance R, and ideal battery of emf E are wired in series. A switch in the circuit is closed at time 0, at which time the current is zero. At any later time t the potential di erence across the resistor is given by: A. E(1 e Lt/R ) B. Ee Lt/R C. E(1 + e Rt/L ) D. Ee Rt/L E. E(1 e Rt/L ) ans: E 62. An 8.0-mH inductor and a 2.0- resistor are wired in series to an ideal battery. A switch in the circuit is closed at time 0, at which time the current is zero. The current reaches half its final value at time: A. 2.8 ms B. 4.0 ms C. 3 s D. 170 s E. 250 s ans: A 448 Chapter 30: INDUCTION AND INDUCTANCE 63. An 8.0-mH inductor and a 2.0- resistor are wired in series to a 20-V ideal battery. A switch in the circuit is closed at time 0, at which time the current is zero. After a long time the current in the resistor and the current in the inductor are: A. 0, 0 B. 10 A, 10 A C. 2.5 A, 2.5 A D. 10 A, 2.5 A E. 10 A, 0 ans: B 64. An 8.0-mH inductor and a 2.0- resistor are wired in series to a 20-V ideal battery. A switch in the circuit is closed at time 0, at which time the current is zero. Immediately after the switch is thrown the potential di erences across the inductor and resistor are: A. 0, 20 V B. 20 V, 0 C. 10 V, 10 V D. 16 V, 4 V E. unknown since the rate of change of the current is not given ans: B 65. An inductor with inductance L resistor with resistance R are wired in series to an ideal battery with emf E. A switch in the circuit is closed at time 0, at which time the current is zero. A long time after the switch is thrown the potential di erences across the inductor and resistor: A. 0, E B. E, 0 C. E/2, E/2 D. (L/R)E, (R/L)E E. cannot be computed unless the rate of change of the current is given ans: A 66. If both the resistance and the inductance in an LR series circuit are doubled the new inductive time constant will be: A. twice the old B. four times the old C. half the old D. one-fourth the old E. unchanged ans: E Chapter 30: INDUCTION AND INDUCTANCE 449 67. When the switch S in the circuit shown is closed, the time constant for the growth of current in R2 is: .. ..... ... .... ...... ...... ...... . . . ............................... .. .................................................. .. .... ... .... ... .... .... ...................... ... ..... ........ ........ ........ ..... .. ...... ..... ...... ...... ... .... ... ... ... .. ... .. ... ... ... ... .. .. ... . . ..... ......... . .... . . ... ......... . ............ ...... . . . . . . . . . . 1 ............. 2 ............. .... ..... ....... ... ... .. .... ... .. .... ... .. .. ... ... .... ... .. .. .. . .................................................................................................................................................................................................................... S L R A. B. C. D. E. R L/R1 L/R2 L/(R1 + R2 ) L(R1 + R2 )/(R1 R2 ) (L/R1 + L/R2 )/2 ans: B 68. The diagrams show three circuits with identical batteries, identical inductors, and identical resistors. Rank them according to the current through the battery just after the switch is closed, from least to greatest. .. .. .. .. .... ... .... ... .... .. .. .. .. ....... ... . ............................ .. . . . . . . . . . . .................... .. .............................. . ... ...... .. ... .... •..... • A. B. C. D. E. 450 .. .. .. .. .... ... .... ... .... .. .. .. .. .. .. .. ......................... .. .. .. 1 .......... ...... ........... ..... ........... .. ..... .. ... .... •..... • 2 3, 2, 1 2 and 3 ties, then 1 1, 3, 2 1, 2, 3 3, 1, 2 ans: C Chapter 30: INDUCTION AND INDUCTANCE .. .. .. .. .... ... .... ... .... .. .. .. .. ....... ... . ............................ .. . . . . . . . . . . .................... .. .............................. . ... ...... .......... ...... ........... ..... ........... .. ..... .. ... .... •..... • 3 ....... ... . ............................ .. . . . . . . . . . . .................... .. .............................. . ... ...... 69. Immediately after switch S in the circuit shown is closed, the current through the battery is: .. .. .. ................................. .......................................................................................................... .. ... .. .. .. .. . ... .... ... .. ... .... ... ... ... ........ 1 ... ... ... .. ... . ..................... ........... . ... ......... .. ...... .. .. . . . . . . . . . ............ . . . . . . .................. .. 2 ................... .. ... . ................................ ....... ... ... ... ... .. . ... . . . . ... . . .. .... .... ... .... .... ... ... .... ... .. ... ... . .... . . . . . . .......................................... ........................................................................................................ R V0 R L S A. B. C. D. E. 0 V0 /R1 V0 /R2 V0 /(R1 + R2 ) V0 (R1 + R2 )/(R1 R2 ) ans: D 70. A 3.5-mH inductor and a 4.5-mH inductor are connected in series. The equivalent inductance is: A. 2.0 mH B. 0.51 mH C. 0.13 mH D. 1.0 mH E. 8.0 mH ans: E 71. A 3.5-mH inductor and a 4.5-mH inductor are connected in series and a time varying current is established in them. When the total emf of the combination is 16 V, the emf of the larger inductor is: A. 7.0 V B. 9.0 V C. 2.3 V D. 28 V E. 36 V ans: B 72. A 3.5-mH inductor and a 4.5-mH inductor are connected in parallel. The equivalent inductance is: A. 2.0 mH B. 0.51 mH C. 0.13 mH D. 1.0 mH E. 8.0 mH ans: A Chapter 30: INDUCTION AND INDUCTANCE 451 73. A 3.5-mH inductor and a 4.5-mH inductor are connected in parallel. When the total emf of the combination is 16 V, the rate of change of the current in the larger inductor is: A. 2.0 × 103 A/s B. 3.6 × 103 A/s C. 4.6 × 103 A/s D. 7.0 × 103 A/s E. 8.1 × 103 A/s ans: B 74. An inductor with inductance L and an inductor with inductance 2L are connected in parallel. When the rate of change of the current in the larger inductor is 1200 A/s the rate of change of the current in the smaller inductor is: A. 400 A/s B. 1200 A/s C. 1600 A/s D. 2000 A/s E. 2400 A/s ans: E 75. The stored energy in an inductor: A. depends, in sign, upon the direction of the current B. depends on the rate of change of current C. is proportional to the square of the inductance D. has units J/H E. none of the above ans: E 76. An inductance L and a resistance R are connected in series to an ideal battery. A switch in the circuit is closed at time 0, at which time the current is zero. The energy stored in the inductor is a maximum: A. just after the switch is closed B. at the time t = L/R after the switch is closed C. at the time t = L/R after the switch is closed D. at the time t = 2L/R after the switch is closed E. a long time after the switch is closed ans: E 77. An inductance L and a resistance R are connected in series to an ideal battery. A switch in the circuit is closed at time 0, at which time the current is zero. The rate of increase of the energy stored in the inductor is a maximum: A. just after the switch is closed B. at the time t = L/R after the switch is closed C. at the time t = L/R after the switch is closed D. at the time t = (L/R) ln 2 after the switch is closed E. a long time after the switch is closed ans: D 452 Chapter 30: INDUCTION AND INDUCTANCE 78. In each of the following operations, energy is expended. The LEAST percentage of returnable electrical energy will be yielded by: A. charging a capacitor B. charging a storage battery C. sending current through a resistor D. establishing a current through an inductor E. moving a conducting rod through a magnetic field ans: C 79. A current of 10 A in a certain inductor results in a stored energy of 40 J. When the current is changed to 5 A in the opposite direction, the stored energy changes by: A. 20 J B. 30 J C. 40 J D. 50 J E. 60 J ans: B 80. A 6.0-mH inductor is in a series circuit with a resistor and an ideal battery. At the instant the current in the circuit is 5.0 A the energy stored in the inductor is: A. 0 B. 7.5 × 10 2 J C. 15 × 10 2 J D. 30 × 10 2 J E. unknown since the rate of change of the current is not given ans: B 81. A 6.0-mH inductor is in a circuit. At the instant the current is 5.0 A and its rate of change is 200 A/s, the rate with which the energy stored in the inductor is increasing is: A. 7.5 × 10 2 W B. 120 W C. 240 W D. 3.0 W E. 6.0 W ans: E 82. A 6.0-mH inductor and a 3.0- resistor are wired in series to a 12-V ideal battery. A switch in the circuit is closed at time 0, at which time the current is zero. 2.0 ms later the energy stored in the inductor is: A. 0 B. 2.5 × 10 2 J C. 1.9 × 10 2 J D. 3.8 × 10 2 J E. 9.6 × 10 3 J ans: C Chapter 30: INDUCTION AND INDUCTANCE 453 83. The quantity B 2 /µ0 has units of: A. J B. J/H C. J/m D. J/m3 E. H/m3 ans: D 84. A 0.20-cm radius cylinder, 3.0 cm long, is wrapped with wire to form an inductor. At the instant the magnetic field in the interior is 5.0 mT the energy stored in the field is about: A. 0 B. 3.8 × 10 6 J C. 7.5 × 10 6 J D. 7.5 × 10 4 J E. 9.9 J ans: B 85. In the diagram, assume that all the magnetic field lines generated by coil 1 pass through coil 2. Coil 1 has 100 turns and coil 2 has 400 turns. Then: ........................................... .......................................... ... .. ... .... ... ... . ... ......... ... .. ... . ... ........................... ....... .. .... . . . . . . .. . . . . . . . . ................... .... ... .. ............................... ... .. ... ... . . .... . . ..... .. .... ... ... . ... . . . ... .... ... .. ... ............................................ ............................................... #1 ............................................................................................. ... ... .... ... ... ... . . ... ..... . ... . .. .... ................. . . ......... . . . . . . . .... . . .... . .... ............ . ... .. ................ . . ... .................. ... ... ... ............. ... ..... . .................. .................. ... ... ... ........ .... ..... .. ... .. ... .. . . .......................................................................................................................... #2 G S A. B. C. D. E. 454 the power supplied to coil 1 is equal to the power delivered by coil 2 the emf around coil 1 will be one-fourth the emf around coil 2 the current in coil 1 will be one-fourth the current in coil 2 the emfs will be the same in the two coils none of the above ans: E Chapter 30: INDUCTION AND INDUCTANCE Chapter 31: ELECTROMAGNETIC OSCILLATIONS AND ALTERNATING CURRENT 1. A charged capacitor and an inductor are connected in series. At time t = 0 the current is zero, but the capacitor is charged. If T is the period of the resulting oscillations, the next time after t = 0 that the current is a maximum is: A. T B. T /4 C. T /2 D. T E. 2T ans: B 2. A charged capacitor and an inductor are connected in series. At time t = 0 the current is zero, but the capacitor is charged. If T is the period of the resulting oscillations, the next time after t = 0 that the charge on the capacitor is a maximum is: A. T B. T /4 C. T /2 D. T E. 2T ans: C 3. A charged capacitor and an inductor are connected in series. At time t = 0 the current is zero, but the capacitor is charged. If T is the period of the resulting oscillations, the next time after t = 0 that the voltage across the inductor is a maximum is: A. T B. T /4 C. T /2 D. T E. 2T ans: C 4. A charged capacitor and an inductor are connected in series. At time t = 0 the current is zero, but the capacitor is charged. If T is the period of the resulting oscillations, the next time after t = 0 that the energy stored in the magnetic field of the inductor is a maximum is: A. T B. T /4 C. T /2 D. T E. 2T ans: B Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 455 5. A charged capacitor and an inductor are connected in series. At time t = 0 the current is zero, but the capacitor is charged. If T is the period of the resulting oscillations, the next time after t = 0 that the energy stored in the electric field of the capacitor is a maximum is: A. T B. T /4 C. T /2 D. T E. 2T ans: C 6. A capacitor in an LC oscillator has a maximum potential di erence of 15 V and a maximum energy of 360 µJ. At a certain instant the energy in the capacitor is 40 µJ. At that instant what is the potential di erence across the capacitor? A. zero B. 5 V C. 10 V D. 15 V E. 20 V ans: B 7. Which of the following has the greatest e ect in decreasing the oscillation frequency of an LC circuit? Using instead: A. L/2 and C/2 B. L/2 and 2C C. 2L and C/2 D. 2L and 2C E. none of these ans: D 8. We desire to make an LC circuit that oscillates at 100 Hz using an inductance of 2.5 H. We also need a capacitance of: A. 1 F B. 1 mF C. 1 µF D. 100 µF E. 1 pF ans: C 9. An LC circuit consists of a 1-µF capacitor and a 4 mH inductor. Its oscillation frequency is approximately: A. 0.025 Hz B. 25 Hz C. 60 Hz D. 2500 Hz E. 15, 800 Hz ans: D 456 Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 10. An A. B. C. D. E. LC circuit has an oscillation frequency of 105 Hz. If C = 0.1 µF, then L must be about: 10 mH 1 mH 25 µH 2.5 µH 1 pH ans: C 11. In the circuit shown, switch S is first pushed up to charge the capacitor. When S is then pushed down, the current in the circuit will oscillate at a frequency of: .................................................................................. ................................................................................ .. ... .... ... ... ... ... ... ... ... ... ... . 0 ... ... ..... . . ...... . . . . .......................................... ................................................................................................................. . . . .... ... ... .. . .... .... ... ... ... ... ... ... ... ... . . . . . . ........ ......... ......... . . . . ........................................................................................ .. ... .. ... .. ... .................................................................... .. .... ...... ...... ...... ... ......... ......... ......... ......... 5 µF V • 50 mH A. B. C. D. E. 318 Hz 0.01 Hz 12.500 Hz 2000 Hz depends on V0 ans: A • • S 12. Radio receivers are usually tuned by adjusting the capacitor of an LC circuit. If C = C1 for a frequency of 600 kHz, then for a frequency of 1200 kHz one must adjust C to: A. C1 /2 B. C1 /4 C. 2C1 D. 4C1 E. 2C1 ans: B 13. An LC series circuit with an inductance L and a capacitance C has an oscillation frequency f . Two inductors, each with inductance L, and two capacitors, each with capacitance C, are all wired in series and the circuit is completed. The oscillation frequency is: A. f /4 B. f /2 C. f D. 2f E. 4f ans: C Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 457 14. The electrical analog of a spring constant k is: A. L B. 1/L C. C D. 1/C E. R ans: D 15. Consider the mechanical system consisting of two springs and a block, as shown. Which one of the five electrical circuits (A, B, C, D, E) is the analog of the mechanical system? \ \ \ \ \ \ \ \ \ \ \ \ \ \ .............. .............................. .............. \ \ .... ...... ...... ...... ................ . . . . . . . . . . . . . . . . . . . . . . \ \ • ................................................. • m • ................................................. • \ \ \ \ \ \ \ \ \ \ k1 k2 \ \ \ \ \ \ \ \ ....... ....... ....... .............................. .... ... .... ... .... ... ............................ ......................... .......................... .... ..... ..... ..... .... ... ... ......... .......... .......... .......... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... .. .... ................................................................................................................................................. ....... ....... ....... .............................. .... ... .... ... .... ... ....................................................................................................... .... ..... ..... ..... .... ... ... ... ......... .......... .......... .......... ... ... ... ... ... ... .. ... ... . ... ... ... ... .... ..... ..... ... .... .... ... .. .. .... ................................................................................................................................................. .... .... .... ...................................... ..... .... ..... .... ........................................ ......................... .......................... ... .... .... .... ... .... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... .. .... ................................................................................................................................................. A B C .... .... .... ..................................... ..... .... ..... .... ................................................................................................................... ... .... .... .... .. ... .. ... ... ... ... ... .. ..... ... . . ... ... ... ... ... ... .... ... .... .... ... ... ... .... . ................................................................................................................................................. .. .. .. .. .. .. ..... ..... ..... ..... ..... ..... .............................. ... ... ... ... ... ... ........................... .......................... .... ..... .... ... .... ... .............................. .... ..... ..... ..... ... .... .... .... .... ... .. ... .......................................... .......................................... .. ... ... ... .... ... ... ... ... ... ... ... ... ... ... ... ... .... . . .......................................................................................................................................................................................... D E ans: A 16. A 150-g block on the end of a spring with a spring constant of 35 N/m is pulled aside 25 cm and released from rest. In the electrical analog the initial charge on the capacitor is: A. 0.15 C B. 6.67 C C. 0.025 C D. 40 C E. 35 C ans: C 458 Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 17. A 150-g block on the end of a spring with a spring constant of 35 N/m is pulled aside 25 cm and released from rest. In the electrical analog the maximum charge on the capacitor is 0.25 C. The maximum current in the LC circuit is: A. 0.38 A B. 0.025 A C. 40 A D. 2.3 A E. 5.3 A ans: A 18. A capacitor in an LC oscillator has a maximum potential di erence of 15 V and a maximum energy of 360 µJ. At a certain instant the energy in the capacitor is 40 µJ. At that instant what is the potential di erence across the capacitor? A. zero B. 5 V C. 10 V D. 15 V E. 20 V ans: B 19. A capacitor in an LC oscillator has a maximum potential di erence of 15 V and a maximum energy of 360 µJ. At a certain instant the energy in the capacitor is 40 µJ. At that instant what is the emf induced in the inductor? A. zero B. 5 V C. 10 V D. 15 V E. 20 V ans: C 20. In an oscillating LC circuit, the total stored energy is U . The maximum energy stored in the capacitor during one cycle is: A. U/2 B. U/ 2 C. U D. U/(2 ) E. U/ ans: C 21. In an oscillating LC circuit, the total stored energy is U and the maximum charge on the capacitor is Q. When the charge on the capacitor is Q/2, the energy stored in the inductor is: A. U/2 B. U/4 C. (4/3)U D. 3U/2 E. 3U/4 ans: E Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 459 22. The total energy in an LC circuit is 5.0 × 10 A. 0.82 µC B. 8.5 µC C. 12 µC D. 17 µC E. 24 µC ans: C 6 J. If C = 15 µF the charge on the capacitor is: 23. The total energy in an LC circuit is 5.0 × 10 A. 10 mA B. 14 mA C. 20 mA D. 28 mA E. 40 mA ans: C 6 J. If L = 25 mH the maximum current is: 24. At time t = 0 the charge on the 50-µF capacitor in an LC circuit is 15 µC and there is no current. If the inductance is 20 mH the maximum current is: A. 15 nA B. 15 µA C. 6.7 mA D. 15 mA E. 15 A ans: D 25. An LC circuit has an inductance of 20 mH and a capacitance of 5.0 µF. At time t = 0 the charge on the capacitor is 3.0 µC and the current is 7.0 mA. The total energy is: A. 4.1 × 10 7 J B. 4.9 × 10 7 J C. 9.0 × 10 7 J D. 1.4 × 10 6 J E. 2.8 × 10 6 J ans: D 26. An LC circuit has a capacitance of 30 µF and an inductance of 15 mH. At time t = 0 the charge on the capacitor is 10 µC and the current is 20 mA. The maximum charge on the capacitor is: A. 8.9 µC B. 10 µC C. 12 µC D. 17 µC E. 24 µC ans: D 460 Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 27. An LC circuit has an inductance of 15 mH and a capacitance of 10 µF. At one instant the charge on the capacitor is 25 µC. At that instant the current is changing at the rate of: A. 0 B. 1.7 × 10 8 A/s C. 5.9 × 10 3 A/s D. 3.8 × 10 2 A/s E. 170 A/s ans: E 28. An LC circuit has a capacitance of 30 µF and an inductance of 15 mH. At time t = 0 the charge on the capacitor is 10 µC and the current is 20 mA. The maximum current is: A. 18 mA B. 20 mA C. 25 mA D. 35 mA E. 42 mA ans: C 29. The graphs show the total electromagnetic energy in two RLC circuits as functions of time. Which of the following statements might be true? E .... ...... ....... ... .... ... ...... ... ...... ... ....... .... ....... .. 1 .... ..... ............... ....... ............ ... . 2 ............................ ......... ................ t A. B. C. D. E. Circuit Circuit Circuit Circuit Circuit ans: A 1 1 1 1 1 has has has has has a smaller resistance and a larger inductance a larger resistance and a smaller inductance the same resistance and a larger inductance a larger resistance and a larger capacitance the same resistance and a smaller capacitance 30. An RLC circuit has a resistance of 200 and an inductance of 15 mH. Its oscillation frequency is 7000 Hz. At time t = 0 the current is 25 mA and there is no charge on the capacitor. After five complete cycles the current is: A. zero B. 1.8 × 10 6 A C. 2.1 × 10 4 A D. 2.3 × 10 3 A E. 2.5 × 10 2 A ans: C Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 461 31. An RLC circuit has an inductance of 25 mH and a capacitance of 5.0 µF. The charge on the capacitor does NOT oscillate but rather decays exponentially to zero. The resistance in the circuit must be: A. greater than or equal to 20, 000 B. less than 20, 000 but greater than 10, 000 C. less than 10, 000 but greater than 5, 000 D. less than 5, 000 but greater than 0 E. 0 ans: A 32. A series circuit with an inductance of 15 mH, a capacitance of 35 µF, and a resistance of 5.0 contains a sinusoidal source of emf with a frequency of 500 Hz. The frequency with which the charge on the capacitor oscillates is: A. 500 Hz B. 1.4 kHz C. greater than 1.4 kHz D. less than 500 Hz E. between 500 Hz and 1.4 kHz ans: A 33. The rapid exponential decay in just a few cycles of the charge on the plates of capacitor in an RLC circuit might be due to: A. a large inductance B. a large capacitance C. a small capacitance D. a large resistance E. a small resistance ans: D 34. An RLC circuit has a capacitance of 12 µF, an inductance of 25 mH, and a resistance of 60 . The current oscillates with an angular frequency of: A. 1.2 × 103 rad/s B. 1.4 × 103 rad/s C. 1.8 × 103 rad/s D. 2.2 × 103 rad/s E. 2.6 × 103 rad/s ans: B 35. The angular frequency of a certain RLC series circuit is 0 . A source of sinusoidal emf, with angular frequency 2 , is inserted into the circuit. After transients die out the angular frequency of the current oscillations is: A. 0 /2 B. 0 C. 2 0 D. 1.5 0 E. 3 0 ans: C 462 Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 36. The angular frequency of a certain RLC series circuit is 0 . A source of sinusoidal emf, with angular frequency , is inserted into the circuit and is varied while the amplitude of the source is held constant. For which of the following values of is the amplitude of the current oscillations the greatest? A. 0 /5 B. 0 /2 C. 0 D. 2 0 E. None of them (they all produce the same current amplitude) ans: C 37. An RLC circuit has a sinusoidal source of emf. The average rate at which the source supplies energy is 5 nW. This must also be: A. the average rate at which energy is stored in the capacitor B. the average rate at which energy is stored in the inductor C. the average rate at which energy is dissipated in the resistor D. twice the average rate at which energy is stored in the capacitor E. three times the average rate at which energy is stored in the inductor ans: C 38. In a purely capacitive circuit the current: A. leads the voltage by one-fourth of a cycle B. leads the voltage by one-half of a cycle C. lags the voltage by one-fourth of a cycle D. lags the voltage by one-half of a cycle E. is in phase with the potential di erence across the plates ans: A 39. In a purely resistive circuit the current: A. leads the voltage by one-fourth of a cycle B. leads the voltage by one-half of a cycle C. lags the voltage by one-fourth of a cycle D. lags the voltage by one-half of a cycle E. is in phase with the voltage ans: E 40. In a purely inductive circuit, the current lags the voltage by: A. zero B. one-fourth of a cycle C. one-half of a cycle D. three-fourths of a cycle E. one cycle ans: B Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 463 41. A series RL circuit is connected to an emf source of angular frequency . The current: A. leads the applied emf by tan 1 ( L/R) B. lags the applied emf by tan 1 ( L/R) C. lags the applied emf by tan 1 ( R/L) D. leads the applied emf by tan 1 ( R/L) E. is zero ans: B 42. A series RC circuit is connected to an emf source having angular frequency . The current: A. leads the source emf by tan 1 (1/ CR) B. lags the source emf by tan 1 (1/ CR) C. leads the source emf by tan 1 ( CR) D. lags the source emf by tan 1 ( CR) E. leads the source emf by /4 ans: A 43. In an RLC series the voltage by 45 A. R = 1/ C B. R = 1/ L C. R = L 1/ D. R = C 1/ E. L = 1/ C ans: C circuit, which is connected to a source of emf Em cos( t), the current lags if: L C C L 44. A coil has a resistance of 60 A. 40 B. 60 C. 80 D. 117 E. 160 ans: C and an impedance of 100 . Its reactance, in ohms, is: 45. The reactance in ohms of a 35-µF capacitor connected to a 400-Hz generator is: A. 0 B. 0.014 C. 0.088 D. 11 E. 71 ans: D 464 Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 46. A 35-µF capacitor is connected to a source of sinusoidal emf with a frequency of 400 Hz and a maximum emf of 20 V. The maximum current is: A. 0 B. 0.28 A C. 1.8 A D. 230 A E. 1400 A ans: C 47. A 45-mH inductor is connected to a source of sinusoidal emf with a frequency of 400 Hz and a maximum emf of 20 V. The maximum current is: A. 0 B. 0.18 A C. 1.1 A D. 360 A E. 2300 A ans: B 48. The impedance of an RLC series circuit is definitely increased if: A. C decreases B. L increases C. L decreases D. R increases E. R decreases ans: D 49. An is: A. B. C. D. E. RLC series circuit has R = 4 , XC = 3 , and XL = 6 . The impedance of this circuit 5 7 9.8 13 7.8 ans: A Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 465 50. The impedance of the circuit shown is: 50 0.20 H 150 µF . .. .. .. ...... ...... ..... ...... ...... ...... ..................... ..... .... ..... .... ..... ...................... .... ..... .... ... .... ... ....................................... ...................................... .. ... ...... .... ...... ...... ....... .... ...... ...... ..... ......... ......... ......... ......... ... .. ... .... ... .. ... ... ... ... .... ... .. ... ... ... .... ... .. ... ........................ . . . . ... . .... ... .... ... . ... ... .. .... ......... .... .. . . . . . . . . .................................................................................................................... ... ....................................................................................................................... . . . . ... ..... . . ... . . . .... . . .. ....... ..................... 50 Hz, 240 Vrms A. B. C. D. E. 51. An 40 A. B. C. D. E. 21 50 63 65 98 ans: D electric motor, under load, has an e ective resistance of 30 and an inductive reactance of . When powered by a source with a maximum voltage of 420 V, the maximum current is: 6.0 A 8.4 A 10.5 A 12.0 A 14.0 A ans: B 52. An RL series circuit is connected to an ac generator with a maximum emf of 20 V. If the maximum potential di erence across the resistor is 16 V, then the maximum potential di erence across the inductor is: A. 2 V B. 4 V C. 12 V D. 25.6 V E. 36 V ans: C 53. When the amplitude of the oscillator in a series RLC circuit is doubled: A. the impedance is doubled B. the voltage across the capacitor is halved C. the capacitive reactance is halved D. the power factor is doubled E. the current amplitude is doubled ans: E 466 Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 54. When the frequency of the oscillator in a series RLC circuit is doubled: A. the capacitive reactance is doubled B. the capacitive reactance is halved C. the impedance is doubled D. the current amplitude is doubled E. the current amplitude is halved ans: B 55. In an RLC series circuit, the source voltage is leading the current at a given frequency f . If f is lowered slightly, then the circuit impedance will: A. increase B. decrease C. remain the same D. need to know the amplitude of the source voltage E. need to know whether the phase angle is larger or smaller than 45 ans: B 56. In the diagram, the function y(t) = ym sin( t) is plotted as a solid curve. The other three curves have the form y(t) = ym sin( t + ), where is between /2 and + /2. Rank the curves according to the value of , from the most negative to the most positive. y(t) . .... ................ ......................... ... .... ..... .... . .. .. .... .... ............... ... ...... .............. . . .. ... ... ... ...... ....... .. ... .. ...... .... .. ..... . ... ..... .. . ... ..... ...... ... .. . .. ... ... .... ..... .. .. .... . ... 3 ... . .. .. ... 1 2 ... ... . . . ... ... ... . . . . ... ... ... .. ... .. t ... .. ... .. ... . .. . ... ... A. B. C. D. E. 1, 2, 3 2, 3, 1 3, 2, 1 1, 3, 2 2, 1, 3 ans: D Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 467 57. An RLC series circuit has L = 100 mH and C = 1 µF. It is connected to a 1000-Hz source and the source emf is found to lead the current by 75 . The value of R is: A. 12.6 B. 126 C. 175 D. 1750 E. 1810 ans: B 58. An RLC series circuit is driven by a sinusoidal emf with angular frequency d . If d is increased without changing the amplitude of the emf the current amplitude increases. If L is the inductance, C is the capacitance, and R is the resistance, this means that: A. d L > 1/ d C B. d L < 1/ d C C. d L = 1/ d C D. d L > R E. d L < R ans: B 59. In a sinusoidally driven series RLC circuit, the inductive reactance is XL = 200 , the capacitive reactance is XC = 100 , and the resistance is R = 50 . The current and applied emf would be in phase if: A. the resistance is increased to 100 , with no other changes B. the resistance is increased to 200 , with no other changes C. the inductance is reduced to zero, with no other changes D. the capacitance is doubled, with no other changes E. the capacitance is halved, with no other changes ans: E 60. In a sinusoidally driven series RLC circuit the current lags the applied emf. The rate at which energy is dissipated in the resistor can be increased by: A. decreasing the capacitance and making no other changes B. increasing the capacitance and making no other changes C. increasing the inductance and making no other changes D. increasing the driving frequency and making no other changes E. decreasing the amplitude of the driving emf and making no other changes ans: A 61. An A. B. C. D. E. RLC series circuit, connected to a source E, is at resonance. Then: the voltage across R is zero the voltage across R equals the applied voltage the voltage across C is zero the voltage across L equals the applied voltage the applied voltage and current di er in phase by 90 ans: B 468 Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 62. An RLC series circuit is connected to an oscillator with a maximum emf of 100 V. If the voltage amplitudes VR , VL , and VC are all equal to each other, then VR must be: A. 33 V B. 50 V C. 67 V D. 87 V E. 100 V ans: E 63. A resistor, an inductor, and a capacitor are connected in parallel to a sinusoidal source of emf. Which of the following is true? A. The currents in all branches are in phase. B. The potential di erences across all branches are in phase. C. The current in the capacitor branch leads the current in the inductor branch by one-fourth of a cycle D. The potential di erence across the capacitor branch leads the potential di erence across the inductor branch by one-fourth of a cycle. E. The current in the capacitor branch lags the current in the inductor branch by one-fourth of a cycle. ans: B 64. The rms value of an ac current is: A. its peak value B. its average value C. that steady current that produces the same rate of heating in a resistor as the actual current D. that steady current that will charge a battery at the same rate as the actual current E. zero ans: C 65. The rms value of a sinusoidal voltage is V0 / 2, where V0 is the amplitude. What is the rms value of its fully rectified wave? Recall that Vrect (t) = |V (t)|. V0 A. B. C. D. E. .................. ... .... . . . ... .. .. ... .... ... . . ...... ...... ...... V0 t .................. ................... ... .... ... ... . . . ..... .. .. t V02 / 2 V02 /2 2V0 V0 / 2 V0 /(2 2) ans: D Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 469 66. A sinusoidal voltage V (t) has an rms value of 100 V. Its maximum value is: A. 100 V B. 707 V C. 70.7 V D. 141 V E. 200 V ans: D 67. An ac generator produces 10 V (rms) at 400 rad/s. It is connected to a series RL circuit (R = 17.3 , L = 0.025 H). The rms current is: A. 0.50 A and leads the emf by 30 B. 0.71 A and lags the emf by 30 C. 1.40 A and lags the emf by 60 D. 0.50 A and lags the emf by 30 E. 0.58 A and leads the emf by 90 ans: D 68. An ac generator producing 10 V (rms) at 200 rad/s is connected in series with a 50a 400-mH inductor, and a 200-µF capacitor. The rms current in amperes is: A. 0.125 B. 0.135 C. 0.18 D. 0.20 E. 0.40 ans: B resistor, 69. An ac generator producing 10 V (rms) at 200 rad/s is connected in series with a 50- resistor, a 400-mH inductor, and a 200-µF capacitor. The rms voltage (in volts) across the resistor is: A. 2.5 B. 3.4 C. 6.7 D. 10.0 E. 10.8 ans: C 70. An ac generator producing 10 V (rms) at 200 rad/s is connected in series with a 50- resistor, a 400-mH inductor, and a 200-µF capacitor. The rms voltage (in volts) across the capacitor is: A. 2.5 B. 3.4 C. 6.7 D. 10.0 E. 10.8 ans: B 470 Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 71. An ac generator producing 10 V (rms) at 200 rad/s is connected in series with a 50- resistor, a 400-mH inductor, and a 200-µF capacitor. The rms voltage (in volts) across the inductor is: A. 2.5 B. 3.4 C. 6.7 D. 10.0 E. 10.8 ans: E 72. The ideal meters shown read rms current and voltage. The average power delivered to the load is: .......... ..... ....... .. . ............................................................................................... .... .......................................................... ... ... . . .... .................. . .... .... ... .. .. .. .......... .......... ..... ...... ..... ....... .. ............. .... .... . ... ....... . ... . .. ... ... .................... .................... . . ..... .... ... ... .. ... .. .. ......................................................................................................................................................................... I V A. B. C. D. E. unknown load definitely equal to V I definitely more than V I possibly equal to V I even if the load contains an inductor and a capacitor definitely less than V I zero, as is the average of any sine wave ans: C 73. The average power supplied to the circuit shown passes through a maximum when which one of the following is increased continuously from a very low to a very high value? R E, f A. B. C. D. E. . . ..... ....... ....... .. .. .......................................... .. .... ..... .... ..... .... ........................................... .. ... .. ... .. ... .. ... ... .... .... .... ... ... ... ... ... ... ... ... ... ..... . ................. . . . .... . . ... . .... ....... . ... .. .... ... ... ...... ... ... . . .... .. . . . . . .................. ..... .. ... ... ... ... ... ... ... ..... .. ... ................................................................................................................................................................................. C Source emf E R C Source frequency f None of these ans: B Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 471 74. In a series RLC circuit the rms value of the generator emf is E and the rms value of the current is i. The current lags the emf by . The average power supplied by the generator is given by: A. (iE/2) cos B. iE C. i2 /Z D. i2 Z E. i2 R ans: E 75. The units of the power factor are: A. ohm B. watt C. radian D. ohm1/2 E. none of these ans: E 76. A series circuit consists of a 15- resistor, a 25-mH inductor, and a 35-µF capacitor. If the frequency is 100 Hz the power factor is: A. 0 B. 0.20 C. 0.45 D. 0.89 E. 1.0 ans: C 77. The main reason that alternating current replaced direct current for general use is: A. ac generators do not need slip rings B. ac voltages may be conveniently transformed C. electric clocks do not work on dc D. a given ac current does not heat a power line as much as the same dc current E. ac minimizes magnetic e ects ans: B 78. A step-down transformer is used to: A. increase the power B. decrease the power C. increase the voltage D. decrease the voltage E. change ac to dc ans: D 472 Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 79. Iron, rather than copper, is used in the core of transformers because iron: A. can withstand a higher temperature B. has a greater resistivity C. has a very high permeability D. makes a good permanent magnet E. insulates the primary from the secondary ans: C 80. The core of a transformer is made in a laminated form to: A. facilitate easy assembly B. reduce i2 R losses in the coils C. increase the magnetic flux D. save weight E. prevent eddy currents ans: E 81. A generator supplies 100 V to the primary coil of a transformer. The primary has 50 turns and the secondary has 500 turns. The secondary voltage is: A. 1000 V B. 500 V C. 250 V D. 100 V E. 10 V ans: A 82. The resistance of the primary coil of a well-designed, 1 : 10 step-down transformer is 1 . With the secondary circuit open, the primary is connected to a 12 V ac generator. The primary current is: A. essentially zero B. about 12 A C. about 120 A D. depends on the actual number of turns in the primary coil E. depends on the core material ans: A 83. The primary of an ideal transformer has 100 turns and the secondary has 600 turns. Then: A. the power in the primary circuit is less than that in the secondary circuit B. the currents in the two circuits are the same C. the voltages in the two circuits are the same D. the primary current is six times the secondary current E. the frequency in the secondary circuit is six times that in the primary circuit ans: D Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT 473 84. The primary of a 3 : 1 step-up transformer is connected to a source and the secondary is connected to a resistor R. The power dissipated by R in this situation is P . If R is connected directly to the source it will dissipate a power of: A. P/9 B. P/3 C. P D. 3P E. 9P ans: A 85. In an ideal 1 : 8 step-down transformer, the primary power is 10 kW and the secondary current is 25 A. The primary voltage is: A. 25, 600 V B. 3200 V C. 400 V D. 50 V E. 6.25 V ans: B 86. A source with an impedance of 100 is connected to the primary coil of a transformer and a resistance R is connected to the secondary coil. If the transformer has 500 turns in its primary coil and 100 turns in its secondary coil the greatest power will be dissipated in the resistor if R =: A. 0 B. 0.25 C. 4.0 D. 50 E. 100 ans: C 474 Chapter 31: ELECTROMAGNETIC OSCILLATIONS & ALTERNATING CURRENT